Hydrocarbons are organic compounds constituted by carbon and hydrogen, one of their main reservoirs is petroleum, formed geochemically at high temperatures and high pressures. The hydrocarbons include saturated compounds; the alkanes and the cycloalkanes, unsaturated compounds; the alkenes and the cycloalkenes, and aromatic compounds which can be mono or polycyclic. In fact, the aliphatic and aromatic hydrocarbons constitute more than 75% of the majority of crude oils. It is even possible to note the presence of organic nitrogen-, sulphur- or oxygen-containing compounds at a low concentration, the presence of an asphaltic fraction and porphyrins (Bertrand and Mille, 1989).
Petroleum is the main source of energy used by man. Problems follow from this during its distribution. These are problems on the one hand due to transport and storage, the risks of biodegradation of the petroleum and its derivative products. Moreover, transport carried out essentially by sea route, plays a major role in the pollution of the environment. L'Academie Nationale des Sciences and de l'Environnement has estimated that the introduction of petroleum into the environment is 1.7 to 8.8 millions of metric tons, most of which are of anthropgenic origin (Leahy and Cowell, 1990).
The pollution of natural environments by hydrocarbons has become a significant preoccupation. In fact, oil bunkers contain sulphur-containing compounds, such as dibenzothiophene (DBT). Their combustion is the main cause of urban pollution and acid rain. Moreover, the microorganisms present in oil wells corrode steel and jeopardize the safety of operating agents by the production of sulphides in anaerobiosis; what is called “Souring of the deposits”. Also, scientists are endeavouring to understand the mechanisms of chemical and biological degradation of hydrocarbons with a view to putting biotechnological processes in place in order to remedy this environmental problem. In fact, the biodegradation treatment, i.e. natural degradation accelerated by the microorganisms, is advantageous since it is less expensive and less harmful from an ecological point of view than the physico-chemical treatments. It can, moreover lead to a complete mineralization of the hydrocarbons.
The catabolism of hydrocarbons has for a long time been considered as a process which is strictly dependent on oxygen. During this process, the initial stage requires the use of oxygenases (Spormann and Widdel, 2000) It has been possible to envisage the ability of a few bacteria to metabolize hydrocarbons in the absence of oxygen only for some twenty years (Atlas, 1981; Bertrand & Mille, 1989; Leahy & Colwell, 1990). The microorganisms capable of such metabolism must have found an alternative in order to degrade hydrocarbons in the absence of oxygen. Up to then, denitrifying bacteria, sulphate-reducing bacteria, and iron-reducing bacteria, capable of degrading hydrocarbons have been isolated.
It was in 1987 that Stetter et al. isolated a new group of sulphate-reducing and hyperthermophilic Archaea from a hydrothermal system on Vulcano Island in Italy (strain Archaeoglobus fulgidus VC 16 (DSM 4304)).
The species belonging to the genus Archaeoglobus are characterized by cells in the form of regular or irregular shells, having a size varying from 0.4 to 1 μm. These are Gram negative, separated or in pairs. These microorganisms tolerate a growth temperature ranging from 60° C. to 95° C., a pH of the order of 5.5-7.5. This are chemoorganotrophs which can oxidize formate, formamide, D− and D+ lactate, glucose, starch, casamino acids, peptone, gelatin, caseine, yeast extract, meat extract and extracts of eubacteria and of archaea cells, in the presence of sulphate, thiosulphate and sulphite as electron acceptors. Moreover, these microorganisms can grow in the presence of H2CO2. In the presence of thiosulphate as an electron acceptor, there is production of sulphides greater than 6 μmol/ml and methane traces of less than 0.1 μmol/ml.
Archaeoglobus fulgidus VC-16 is the typical species; the cells are presented in the form of irregular spheres with an envelope composed of glycoproteins. This strain has an optimum growth temperature equal to 83° C. and a generation time of approximately 4 hours.
This species is filed in the German collection of microorganisms at Braunschweig-Stockheim, referred to as VC-16 (4304).
Klenk et al. (1997) carried out the sequencing of the genome of a. fulgidus, VC-16 while comparing it with an Archaea: Methanococcus jannaschii. In fact, A. fulgidus is the first sulphate-reducing Archaea to have its genome sequenced. The genome of A. fulgidus is a circular chromosome formed by 2,178,400 base pairs, with a G+C composition of the DNA equal to 48.5%.
Sulphate-reduction is the most abundant respiratory process in anoxic marine environments. Sulphate (SO42−) is first to be activated in order to produce Adenylsulphate (Adenosine-5′-phosphosulphate; APS) followed by sulphite. The enzyme involved in the dissimilatory sulphate-reduction process is adenylsulphate reductase which reduces the activated sulphate. The sulphite, thus formed, is reduced to sulphide by the action of a desulphoviridin, sulphite reductase.
Although it has been pointed out that A. fulgidus is incapable of growing on acetate, several acetyl-CoA synthetase genes have been detected in its genome, They are responsible for the conversion of acetate to acetyl-CoA. The presence of 57 enzymes involved in the β-oxidation suggests that A. fulgidus is capable of oxidizing the fatty acids. The β-oxidation route in Archaea is similar to that: talking place in the Bacteria and mitochondria.
In A. fulgidus, the production of traces of methane during growth is probably due to a reduction of N5-methyltetrahydromethanopterin via carbon monoxide dehydrogenase. Moreover, it has been suggested that A. fulgidus contains a type of CO dehydrogenase similar to that of Rhodospirillum rubrum allowing it to grow by using carbon monoxide as sole source of energy.